Atom absorption spectroscopy method and atom absorption spectrometer

ABSTRACT

Atom absorption spectroscopy method generates at least one first beam of monochromatic radiation at a certain wavelength. Radiation is separated into a test beam and a reference beam, and only the test beam is led through a test area. Subsequently, both beams pass through a slot system into a monochromator, and after their spectral breakdown, reach a detector system. In order to improve such atom absorption spectroscopy method in a way that allows the test beam and the reference beam as well as their spectra to be guided close to, or adjacent to one other onto the detector system, and, at the same time enables a simple and easy adaptation of the spectrometer&#39;s effective spectral range to a monochromator&#39;s characteristics, the test beam and the reference beam thus pass through different slots in the slot system and are projected onto essentially spatially separate areas of the detector system, with their spectra&#39;s broken-down.

This application claims priority of pending German Application No.10207742.8 filed on Feb. 22, 2002.

The invention relates to atom absorption spectroscopy method andcorresponding atom absorption spectrometer.

A method like this, or alternatively, a corresponding device, is knownfrom EP 0084391. Within this context, a radiation source, such as ahollow cathode lamp or a discharge lamp without electrodes, generatesmonochromatic radiation, which is separated by a beam separator into atest (sample) beam and a reference beam. The test beam is led through atest area in which a sample is atomized by means of a flame. Theatomized sample absorbs part of the radiation, and the quantity ofradiation absorbed represents a qualitative measure (unit ofmeasurement) for the concentration of the element in the sample that isto be analyzed. Following the test area, the test beam is transferredthrough a series of optical elements and reunited with the referencebeam. After that, both beams exit through the slot of a slot system andarrive in a monochromator. In the monochromator, a spectrum of theradiation is created. After leaving the monochromator, the spectrallydissected radiation hits a detector system.

Other atom absorption spectrometers are known, which reunite a test beamand a reference beam through a chopper, while the beams' intensities aredetermined in correspondence to the chopper's frequency.

Another known system consists of atom absorption spectrometer containingmovable mirrors, which either allow a single beam to pass through thetest area or deflect it in order to circumvent it. The reference beams'intensities are therefore measured between the test beams' intensities,with the measuring frequency based on the mirror's movement. It has tobe noted, though, that any change in the mirrors' position results in adirect change of the beams' intensity and therefore leads to a shift inthe base lines.

Finally, it is also known to periodically remove the test area, or,alternatively, the burner mounted in it, including the accompanyingflame, from the orbit of the test beam. With this, however, frequency iscomparatively low and demands an exact control of the burner's movementto avoid changes in sensitivity. Additionally, the measurements of thereference's intensity in the test area might be affected by theoperator's activities.

An additionally known atom absorption spectrometer also operates with atest beam and a reference beam, with both beams being spatiallyseparated. Both beams are led through a slot into a Littrowmonochromator and subsequently hit a detector system. This atomabsorption spectrometer has the advantage of representing acomparatively simple optical system, in which the measuring time can beoptimally utilized, enabling a simultaneous measurement of both beams atthe same time.

With regard to EP 0084391, the patent applied for here is aimed atimproving a known atom absorption spectroscopy method and a known atomabsorption spectrometer in a way that allows the test beam and thereference beam to be guided close to each other and, at the same time,enables their respective spectra to be projected close to each otheronto a detector system. At the same time, this also enables thespectrometer's effective spectral range to be easily adapted to themonochromator's characteristics.

According to this method, this task is fulfilled when the test beam andthe reference beam pass through separate slots of the slot system, arebroken down spectrally, and hit essential spatially separated areas ofthe detector system.

According to the device, this task is fulfilled when the slot systemfeatures at least two slots for the separate penetration of the testbeam and the reference beam, and when the beams' isolated ranges of wavelength are spatially separate when they hit the detector system.

The separate slots for the test beam and the reference beam, and theresulting separate slot sizes for each beam, might be easily utilized inorder to vary the monochromator's effective spectral range. Furthermore,the beams are separated spatially and not in terms of time, so thatcomplicated devices for a chronological separation, such as a chopper orsomething similar, become obsolete.

In order to at least enable a sequential multiple element analysis, thefirst beam might be created, for the purpose of selecting a suitablewavelength, by selecting different radiation sources. These may bearranged on a carousel or carousel-like device and may be positioned ina way that enables the beams they produce to be injected into the atomabsorption spectrometer.

In order to provide different slot sizes, the slot system may possess anumber of choices of slots, especially if these are arranged in pairs,for the separate passage of the test and the reference beams. Afterselection, the according slot pair is brought into position, and theslot size defined by the pair causes a variation of the monochromator'seffective spectral range.

In order to easily position the slot pairs, they may be arranged on apivotal slot disc.

The slot disc may be rotated manually. In order to automatically adjustthe slot disc, it may be attached to a driving device. Such a drivermay, for example, be an electric step-by-step engine, or somethingsimilar to it.

In order to achieve an essentially rectangular arrangement of detectors,it may be useful for the slot to have a rhomboid cross-section.

An optical deflection device for the deflection or diversion of thefirst beam in the direction of the beam separator may be devised inorder to achieve a compact atom absorption spectrometer. This reducesthe physical distance between the corresponding radiation source and thebeam separator.

Within this context, it may also be considered of further advantage toarrange a series of optical elements in the orbit of the test as well asin the orbit of the reference beams for the purpose of deflection and/orfocusing. This too may result in a more compact design of the atomabsorption spectrometer, with the beams being deflected accordingly, andfocused within the spectrometer.

If there is a need to measure either only the test beam's or referencebeam's spectra, the test beam and/or reference beams may be temporarilyinterrupted, especially before passing through the slot system. A beaminterrupter device that is located in a suitable position may achievethis.

A prime example of a simply designed beam interrupter device may be amagnetically operable closing blind.

In order to separate the spectra of the test and reference beams in asimple manner, the monochromator may generate a two-dimensionalspectrum.

A simple and cost effective realization of such a monochromator may beachieved by at least one Echelle grid and one dispersion prism. The veryhigh dispersion of such a monochromator can be used to increasegeometrical beam passage in the atom absorption spectrometer. Thedispersion prism is mounted behind the Echelle grid and its dispersionoccurs orthogonally in relation to the grid. Such a two-dimensionalspectrum enables the two components (the Echelle grid and the dispersionprism) to compensate for each other's positioning errors. Furthermore,the monochromator's effective spectral range may be altered by the sizeof the corresponding slot in a way that the range matches thecharacteristics of the Echelle spectra.

Maximizing crosswise dispersion with the help of the dispersion prismprovides the opportunity to utilize the dark space between the spectra'sdifferent orders to measure the reference spectra. Additionally, themonchromator may be operated with the maximum signal/noise relation inany given situation, since the spectral lines emitted by the radiationsource are comparatively weak.

In order to correct the measuring results in response to changes in thebackground, a second, non-monochromatic beam may be injected into theatom absorption spectrometer for purposes of background compensation,especially in a chronologically shifted manner (at a time difference) inrelation to the first beam. This may be achieved by at least oneadditional radiation source, such as a D₂ radiator (to state oneexample). Such a second beam may be analogous to the first beam, and mayalso be separated into a test beam and a reference beam by means of thebeam separator.

The detector system may possess multiple detection areas with numerousdetector elements, in order to measure different orders of the spectraat the same time, or to at least measure spectra for the test and thereference beams simultaneously.

In order to be able to measure each order of the spectral breakdownprecisely and separately, each detector area may be charged by at leastone order of the spectral breakdown.

Two detector areas may each be arranged in a staggered manner inrelation to the corresponding slot pair, in order to enable a quick andprecise measurement when the slot pairs are projected onto the surfaceof the detector system.

Detector elements may, for example, be semi-conductive photo detectorsor ideally photo diodes. It is also possible to use CCD (charge coupleddevices) or CID (charge injection devices) as semi-conductive photodetectors.

It has already been pointed out that with a maximized cross-wisedispersion of the monochromator, comprised of Echelle grid anddispersion spectra, the dark space between the spectra's orders may beutilized to measure the reference spectra. In this case, the spectra ofthe test beam and the reference beam are projected onto the detectorsystem, spaced in an essentially staggered manner in relation to eachother. Yet the spectra may still overlap, either partially or at leastin the areas of other orders.

BRIEF DESCRIPTION OF THE DRAWINGS

Based on the figures of the attached illustrations, the followingexplains an advantageous version of the invention:

FIG. 1 shows a view from above upon the sample version of atomabsorption spectrometer based on the invention at issue;

FIG. 2 shows a slanted perspective from above onto a slot disc, and

FIG. 3 shows a view from above onto two detector areas

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a view from above the sample version of atom absorptionspectrometer 15 based on the invention. It features a test area 4, inwhich, for example, a flame 42 is located for the purpose of atomizing asample which is to be analyzed. Additionally, the atom absorptionspectrometer 15 contains a number of radiation sources: 12 and 13.Besides the two depicted radiation sources, additional radiation sourcesmay exist. The radiation sources emit an essentially monochromaticprimary radiation 39, which, depending on which radiation source (12 or13) is employed, hits a deflection device 19. On the bottom side thatfaces the radiation sources 12 and 13, the deflection device features anumber of optical deflection means, such as mirrors or prisms. Thesereflect the corresponding primary beam 39 into the direction of a beamseparator 16, in the form of a first beam 1. The beam separator 16separates the first beam 1 into a test beam 2 and a reference beam 3.

By means of a toroid or ring mirror 20, the test beam 2 is deflectedinto the direction of the test area 4 where it is focused in the area ofthe flame 42. Part of the test beam 2 is absorbed within the flame 42,and the test beam with its now weakened intensity leaves the test area 4and is reflected and focused through another toroid or ring mirror 23into the direction of a flat mirror 25.

The flat mirror 25 is mounted on a clamping device 43 in a flexiblemanner. The flat mirror 25 deflects the test beam 2 in the direction ofa slot system 5. After passing through a slot in slot system 5, the testbeam hits a collimator 27. From there, the test beam is deflected intothe direction of a monochromator 6, which consists of an Echelle grid 32and a dispersion prism 33. The dispersion prism 33 is mounted behind theEchelle grid 32 and features, on the side facing away from the Echellegrid 32, a prism mirror 36. The latter can be adjusted by means of anadjustment device 37.

The test beam is projected from the dispersion prism 33 onto anothercollimator 41, and from there is focused on the surface of a detectorsystem 7, consisting of detector areas 10 and 11.

A suitable electronic measuring and evaluation device is mounted on theside facing away from the detector system 7.

The reference beam 3 produced by the radiation separator 16 is deflectedvia a flat mirror 21 onto a toroid or ring mirror 22, which againdeflects it into the direction of a flat mirror 24 where it is focused .From the flat mirror 24, which is also mounted on a clamping device 44in an adjustable manner, the reference beam 3 is deflected into thedirection of yet another flat mirror 26. Finally, the reference beam 3is led from the flat mirror 26 through a slot of the slot system 5 intothe direction of a collimator 27. From there, the reference beam followsa route analogous to the one followed by the test beam 2, i.e., it isled to the monochromator 6 and subsequently, from there via anothercollimator 41 to the detector system 7.

The slot system 5 is designed as a slot disc 17. This disc may berotated through an attached driving device 18.

For purposes of backgound compensation during measurements with thedetector system 7, another radiation source 35 is designed. This sourceemits a second, non-monochromatic beam 14. Following deflection by aflat mirror 40, the second beam 14 is separated into a test beam and areference beam, analogous to the first beam 1. In relation to the firstbeam, the second beam 14 is emitted at a time interval. The spectra ofthe first and second beams, and the spectra of their respective test andreference beams are registered by the detector system 70, atcorresponding time intervals.

The radiation source 35 usually is a D₂ radiation source, and radiationsources 12,13 may consist of discharge lamps without electrodes orhollow cathode lamps.

If it is necessary to interrupt the test and/or reference beams 2 and 3,to measure only one beam through the detector system 7, beam interrupterdevices 28, 29 are positioned in the orbit of the test beams and/or inthe orbit of the reference beams 2 and 3. Such beam interrupter devices28, 29 are designed as a magnetically operable closing blind 30 and 31.

FIG. 2 shows a slanted perspective from above onto a slot disc 17,serving as a slot system 5 following FIG. 1. This disc features acentral drilling hole 45 for the reception of a drive shaft 18,following FIG. 1. In the slot disc 17, the sample design shows fourpairs of slots 8 and 9. One of the slots is assigned to the test beam 2,and the other slot to the reference beam 3. The beams in question eachpass through the slots, the latter seen in FIG. 1 as arranged at theslot disc's 17 top. At the same time, the slots 8 and 9 are positionedin a staggered manner in relation to each other, both in radialdirection and vertically in relation to the radial direction. The slotsof the various slot pairs have different sizes. The cross-section of therespective slots is approximately rhomboid. The slot sizes cause avariation of the effective spectral range of the Echelle spectrometer,which consists of the Echelle grid 32 and the dispersion prism 33.

FIG. 3 shows a simplified view from above onto a detector system 7 withtwo detector areas 10 and 11. Of course it is possible to arrange formore than two detector areas 10 and 11, with each of these detectorareas possessing a multitude of detector elements 34, such as photodiodes, to state one obvious example. In relation to each other, thedetector areas 10 and 11 are positioned analogous to the slot pairs 8and 9. They are also positioned laterally and directed upwards in astaggered manner. For example, detector area 10 is assigned to slot 8and detector area 11 to slot 9. This way, the test and reference beams 2and 3, when passing through the corresponding slot pair 8 and 9, areseparated, and their spectral breakdown can be measured through thedetector areas 10 and 11.

1. An atomic absorption spectroscopy method comprising: separatingquasi-monochromatic radiation into a first test beam and a firstreference beam; directing the first test beam to a test area wherein thefirst test beam interacts with an atomized material sample for analysisof the sample; filtering the first test beam over a first spectralrange; spectrally isolating a segment of the first test beam; filteringthe first reference beam over a second spectral range; spectrallyisolating a segment of the first reference beam; separatingnon-monochromatic radiation into a second test beam and a secondreference beam; directing the second test beam to the test area whereinthe second test beam interacts with an atomized material sample foranalysis of the sample; filtering the second test beam over a thirdspectral range; spectrally isolating a segment of the second test beam;filtering the second reference beam over a fourth spectral range; andspectrally isolating a segment of the second reference beam.
 2. Themethod as set forth in claim 1 wherein filtering the first test beamcomprises low pass filtering the first test beam.
 3. The method as setforth in claim 2 wherein low pass filtering the first test beamcomprises directing the first test beam through a first aperture.
 4. Themethod as set forth in claim 1 wherein filtering the first referencebeam comprises low pass filtering the first reference beam.
 5. Themethod as set forth in claim 4 wherein low pass filtering the firstreference beam comprises directing the first reference beam through asecond aperture.
 6. The method as set forth in claim 1 wherein filteringthe second test beam comprises low pass filtering the second test beam.7. The method as set forth in claim 6 wherein filtering the second testbeam comprises directing the second test beam through a second aperture.8. The method as set forth in claim 1 wherein filtering the secondreference beam comprises low pass filtering the second reference beam.9. The method as set forth in claim 8 wherein low pass filtering thesecond reference beam comprises directing the second reference beamthrough a second aperture.
 10. The method as set forth in claim 1further comprising: generating the quasi-monochromatic radiation; andafter a prescribed time interval in relation to generating thequasi-monochromatic radiation, generating the non-monochromaticradiation.
 11. The method as set forth in claim 1 further comprisingdetecting the spectrally isolated segment of the first test beam. 12.The method as set forth in claim 1 further comprising detecting thespectrally isolated segment of the first reference beam.
 13. The methodas set forth in claim 1 further comprising detecting the spectrallyisolated segment of the second test beam.
 14. The method as set forth inclaim 1 further comprising detecting the spectrally isolated segment ofthe second reference beam.
 15. The method as set forth in claim 3wherein filtering the first test beam over a first spectral rangecomprises varying the first spectral range.
 16. The method as set forthin claim 15 wherein varying the first spectral range comprises varying adimension of the aperture.
 17. The method as set forth in claim 3wherein filtering the first reference beam over a second spectral rangecomprises varying the second spectral range.
 18. The method as set forthin claim 17 wherein varying the second spectral range comprises varyinga dimension of the aperture.
 19. The method as set forth in claim 3wherein filtering the second test beam over a third spectral rangecomprises varying the third spectral range.
 20. The method as set forthin claim 19 wherein varying the third spectral range comprises varying adimension of the aperture.
 21. The method as set forth in claim 3wherein filtering the second reference beam over a fourth spectral rangecomprises varying the fourth spectral range.
 22. The method as set forthin claim 21 wherein varying the fourth spectral range comprises varyinga dimension of the aperture.
 23. An atomic absorption spectroscopymethod comprising: separating quasi-monochromatic radiation into a firsttest beam and a first reference beam; directing the first test beam to atest area wherein the first test beam interacts with an atomizedmaterial sample for analysis of the sample; spatially segregating thefirst test beam and the first reference beam by directing the first testbeam and the first reference beam through spatially segregatedapertures; spectrally isolating a segment of the first test beam;spectrally isolating a segment of the first reference beam; separatingnon-monochromatic radiation into a second test beam and a secondreference beam; directing the second test beam to the test area whereinthe second test beam interacts with an atomized material sample foranalysis of the sample; spatially segregating the second test beam andthe second reference beam; spectrally isolating a segment of the secondtest beam; and spectrally isolating a segment of the second referencebeam.
 24. The method as set forth in claim 23 further comprising varyinga dimension of the apertures.
 25. The method as set forth in claim 23wherein spatially segregating the second test beam and the secondreference beam comprises directing the second test beam and the secondreference beam through a second set of spatially segregated apertures.26. The method as set forth in claim 25 further comprising varying adimension of the second set of apertures.
 27. An atomic absorptionspectrometer comprising: a first source of radiation for generatingfirst radiation; a second source of radiation for generating secondradiation; a device for separating the first radiation into a first testbeam and a first reference beam and for separating the second radiationinto a second test beam and a second reference beam; a system ofapertures receptive of the first test and first reference beams or thesecond test and second reference beams; a spectrum analyzer receptive ofthe first test and first reference beams and the second test and secondreference beams from the system of apertures for spectrally isolating asegment of the first test and first reference beams and a segment of thesecond test and second reference beams; a detector receptive of thefirst test and first reference beams or the second test and secondreference beams from the spectrum analyzer for detecting the spectrallyisolated segment of the first test and first reference beams or thespectrally isolated segment second test and second reference beams. 28.The spectrometer as set forth in claim 27 wherein the first radiation isquasi-monochromatic radiation.
 29. The spectrometer as set forth inclaim 27 wherein the second radiation is non-monochromatic radiation.30. The spectrometer as set forth in claim 27 wherein the system ofapertures comprises a system of spatially separated apertures.
 31. Thespectrometer as set forth in claim 28 wherein a first aperture of thesystem of spatially separated apertures is receptive of a test beam anda second aperture of the system of apertures is receptive of a referencebeam.
 32. The spectrometer as set forth in claim 27 wherein the spectrumanalyzer comprises a monochromator.
 33. The spectrometer as set forth inclaim 32 wherein the monochromator comprises an echelle monocromator.34. The spectrometer as set forth in claim 33 wherein the echellemonochromator comprises: an echelle grid; and a dispersing prism. 35.The spectrometer as set forth in claim 27 wherein the detector comprisesa plurality of spatially separated detection areas.
 36. The spectrometeras set forth in claim 35 wherein the detector is receptive of thespectrally isolated segments of the first and second test beams at afirst detection area and of the spectrally isolated segments of thefirst and second reference beams at a second detection area.
 37. Thespectrometer as set forth in claim 27 further comprising a test areareceptive of an atomized material sample and the first and second testbeams; wherein the first and second test beams interact with thematerial sample for analysis of the sample.
 38. The method as set forthin claim 23 further comprising: generating the quasi-monochromaticradiation; and after a prescribed time interval in relation togenerating the quasi-monochromatic radiation, generating thenon-monochromatic radiation.
 39. The spectrometer as set forth in claim27 wherein generation of first radiation and generation second radiationare sequentially spaced in time.